Polysaccharides constitute the bulk of the plant cell walls and have been traditionally classified into three categories: cellulose, hemicellulose, and pectin. Fry, S. C. (1988), The growing plant cell wall: Chemical and metabolic analysis, New York: Longman Scientific & Technical. Whereas cellulose is made at the plasma membrane and directly laid down into the cell wall, hemicellulosic and pectic polymers are first made in the Golgi apparatus and then exported to the cell wall by exocytosis. Ray, P. M., et al., (1976), Ber. Deutsch. Bot. Ges. Bd. 89, 121-146. The variety of chemical linkages in the pectic and hemicellulosic polysaccharides indicates that there must be tens of polysaccharide synthases in the Golgi apparatus. Darvill et al., (1980), The primary cell walls of flowering plants. In The Plant Cell (N. E. Tolbert, ed.), Vol. 1 in Series: The biochemistry of plants: A comprehensive treatise, eds. P. K. Stumpf and E. E. Conn (New York: Academic Press), pp. 91-162.
Even though sugar and polysaccharide compositions of the plant cell walls have been well characterized, very limited progress has been made toward identification of the enzymes involved in polysaccharides formation, the reason being their labile nature and recalcitrance to solubilization by available detergents. Sporadic claims for the identification of cellulose synthase from plant sources have been made over the years. Callaghan, T., and Benziman, M. (1984), Nature 311, 165-167; Okuda, et al., (1993), Plant Physiol. 101, 1131-1142. However, these claims have been met with skepticism. Callaghan, T., and Benziman, M. (1985), Nature 314, 383-384; Delmer, et al., (1993), Plant Physiol. 103, 307-308. It was only recently that a putative gene for plant cellulose synthase (CelA) was cloned from the developing cotton fibers based on homology to the bacterial gene. Pear, et al., Proc. Natl. Acad. Sci. (USA) 93, 12637-12642; Saxena, et al., (1990), Plant Molecular Biology 15, 673-684; see also, WO 9818949.
Cellulose, by virtue of its ability to form semicrystalline microfibrils, has a very high tensile strength which approaches that of some metals. Niklas, K. J. (1992), Plant Biomechanics: An engineering approach to plant form and function, The University of Chicago Press, p. 607. Bending strength of the culm of normal and brittle-culm mutants of barley has been found to be directly correlated with the concentration of cellulose in the cell wall. Kokubo, et al., (1989), Plant Physiology 91:876-882; Kokubo, et al., (1991) Plant Physiology 97:509-514.
As stalk composition contributes to numerous quality factors important in maize breeding, what is needed in the art are products and methods for manipulating cellulose concentration in the cell wall and thereby altering plant stalk quality to provide, for example, increased standability or improved silage. The present invention provides these and other advantages.